Carbonaceous materials are not only excellent in various mechanical characteristics but also advantageous in that they hardly react with metals. Therefore, they are widely used in the semiconductor, machinery and nuclear industries.
In recent years, the market for silicon semiconductors and compound semiconductors, typically SiC and GaN has rapidly expanded. With such market expansion, the requirement imposed on carbonaceous materials have become severer. In addition, carbonaceous materials are also used in the growth of CaF2 single crystals, which are used for emitting short wavelength excimer laser beams in semiconductor lithography (hereinafter referred to as “photo etching”) for attaining large scale integration of semiconductors.
Enhanced resolution is required in semiconductor photo etching and, for realizing resolution enhancement, CaF2 has come into use for emitting short wavelength excimer laser beams, such as krypton fluoride (248 nm), argon fluoride line (193 nm) nad fluorine (157 nm). In connection with this, lenses made of fluorite (single crystal CaF2) have come into use, since the conventional noncrystalline optical materials cannot transmit light at 193 nm. In the following, the production of CaF2 single crystals is described specifically. CaF2 single crystals are produced by the Bridgman technique of Czochralski (CZ) process. The use of a graphite material as jigs in furnaces, such as heating elements, in the production of CaF2 single crystals by the Bridgman technique, for instance, is described in Laid-open Japanese Patent Application (JP Kobai) No. 2000-137101.
Graphite materials generally contain metal impurities entrapped in pores and between graphite lattices thereof, hence they as such cannot be used in semiconductor manufacturing. Therefore, the applicants have previously proposed, for use in the semiconductor and nuclear industries, high purity graphite materials having a metal impurity (ash) content of 5 ppm or less as a result of treatment of graphite materials with a halogen-containing gas, for instance, for attaining high levels of purity (JP Kokai No. S64(1989)-18964; Japanese Patent Publication (JP Kokoku) No. H06(1994)-35325). They have also recently proposed, in JP Kokai 2002-249376, carbonaceous materials with reduced nitrogen content for use in the manufacture of compound semiconductors.
However, even if such high purity graphite materials reduced in metal impurity content and in nitrogen content as disclosed in the above-cited patent documents are used as jigs in furnace, the yield in the production of CaF2 single crystals becomes very low, less than 10%, because impurities, such as oxygen, chlorine, phosphorus and sulfur, have an unfavorable effect.
An object of the present invention is to provide the high purity carbonaceous material, which have not only reduced oxygen, nitrogen, chlorine, phosphorus and sulfur content in pores, but also reduced amounts of oxygen, nitrogen, chlorine, phosphorus, sulfur and boron which are bound to carbon atoms in graphite material.